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The fate of rainfall

Translational Ecology
by Bill Schlesinger
Mon Sep 17th, 2018

Do you ever wonder what happens to all the rain (and snow) that falls on land? Despite dramatic flash flooding after big storms, studies of the fate of precipitation over long periods and large regions, show that only about 35% of it becomes runoff in streams and rivers. About 65% returns to the atmosphere as water vapor. That is easily seen when puddles evaporate from paved surfaces like parking lots.

When rain falls on lawns, pastures and forests, some evaporates from the surface of the plants, but much of it sinks into the soil. Losses of evaporation from the soil are rather ineffective below about 4” (10 cm) depth. The water that percolates below that level is removed by plants, which take up water in their roots and transpire it from their leaf surfaces—a loss of water vapor that is similar to evaporation but controlled by plants.

Over long periods and large regions, about 60% of the water that is returned to the atmosphere as vapor is lost by transpiration and 40% by evaporation. The sum of these is known as evapotranspiration or ET. In the tropics transpiration can dominate ET, but even in desert regions 40 to 70% of ET can be lost as transpiration through plants. In some of my past work in the deserts of New Mexico, I found that moisture accumulated in the soil below about 4” depth in barren experimental plots.

The losses of water by transpiration have big implications for the impacts of humans on the landscape. When we remove plants, such as by clearing forests, we eliminate the uptake of water from the soil by deep roots, and we lower the loss of water vapor to the atmosphere by transpiration. This results in warmer climate conditions in the region. In many cases, this is so likely to reduce subsequent rainfall, since a large percentage of the rain that falls in a given region is generated by local ET, which is dominated by transpiration. Drought follows the plow.

But, removing plants can increase the runoff of surface water from the landscape, increasing stream flow and the potential for flash floods. We can see this easily in urban areas, where a large portion of the surface has been rendered impermeable by pavement. Normally, plants modulate the turning of the hydrologic cycle by reducing surface runoff, increasing the amount of water that enters the soil, and returning it slowly to the atmosphere by transpiration after storms.

References:

Glantz, W. H.   2007. Drought follows the plow. Cambridge University Press

Schlesinger, W.H. and S. Jasechko. 2014. Transpiration in the global water cycle. Agricultural and Forest Meteorology 189: 115-117.

Wei, Z.W. and 5 others. 2017. Revisiting the contribution of transpiration to global terrestrial evapotranspiration. Geophysical Research Letters 44: 2792-2801.